Controlled addition of promoters to ethylene oxide catalysts

ABSTRACT

An improved method is disclosed for improving the selectivity of supported silver catalysts by incorporating a promoter therein. The improvement comprises forming a solution of a compound of the promoter and an anion selected from the group consisting of unsaturated carboxylic acids, aminoorganic acids, and hydroxybenzoic acids in a solvent and contacting the catalyst with the solution.

BACKGROUND OF THE INVENTION

This invention relates to a method for improving the selectivity ofsupported silver catalysts for the production of ethylene oxide.Specifically, the invention relates to an improvement in the knownprocess for incorporating promoters into such catalysts.

Supported silver-based catalysts have been used industrially for manyyears for the oxidation of ethylene to ethylene oxide with oxygen orair. Most of the ethylene which is reacted is converted into ethyleneoxide on the silver-impregnated catalyst support material and theremainder of the ethylene is converted almost exclusively to carbondioxide and water. The goal is to react as much ethylene as possible,i.e., high productivity, such that the greater amount of the ethylene isconverted to ethylene oxide, i.e., high selectivity.

It is well-known in the art that the incorporation of promoters, such asrubidium or cesium, into these catalysts will increase the selectivitythereof. U.S. Pat. No. 4,012,425, issued Mar. 15, 1977, discloses onesuch process which comprises treating the catalyst with a solution ofcesium or rubidium. There are many similar disclosures in the art, bothfor the manufacture of new catalyst and for the regeneration of spentcatalyst. However, nowhere in the prior art is there any disclosure thatany particular anion should be used with the promoters, other than thatthe anion should not be a catalyst poison such as sulfur-containingcompounds. The above patent states that no unusual effectiveness isobserved with the use of any particular anion and goes on to say thatnitrates, nitrites, chlorides, iodides, bromates, bicarbonates,oxalates, acetates, tartrates, lactates, and isopropoxides may be used.

SUMMARY OF THE INVENTION

The present invention relates to an improvement in the known method forimproving the selectivity of supported silver catalysts by incorporatinga promoter therein. The improvement comprises forming a solution of acompound of the promoter and an anion selected from the group consistingof unsaturated carboxylic acids, aminoorganic acids, and hydroxybenzoicacids in the solvent and then contacting the catalyst with the solution.The solution is drained from the catalyst and the catalyst is dried.

DETAILED DESCRIPTION OF THE INVENTION

The addition of promoters to silver-based ethylene oxide catalysts is anestablished part of the art of catalyst manufacture. Promoters have beenadded to both new and used catalysts to provide increased selectivityand activity. A large array of anions have been listed as suitable foruse with promoters. The chief restriction on the anion is the lack ofharmful effect on catalyst performance.

It has been found that certain promoter-anion combinations are superiorto those routinely employed in catalyst manufacture or regenerationtoday. The anion employed must have two general characterstics: (1)formation of a compound with a promoter which is soluble in a suitablesolvent and (2) it must have one or more functional groups which have anaffinity for the promoter ion and one or more groups which have anaffinity for the silver surface of the catalyst. The solubility of thepromoter-anion combination is not restricted to water or aqueoussystems, but encompasses all non-aqueous solvents that of themselves orin combination are not deleterious to catalyst performance. Suitablesolvents are methanol, water, aliphatic, alicyclic, or aromatic ethers,alcohols, hydrocarbons, and ketones, and aliphatic or aromatic esters,amines, amides, aldehydes, and nitriles.

The polyfunctionality of the anion provides for site specificapplication of the promoter, optimum promoter utilization, and superiorcatalyst performance. The method provides homogeneous application of thepromoter to the catalyst and minimizes the macro and microscopicconcentration variations which adversely effect catalyst performance.The homogeneity is provided by the affinity of the functional groups forthe silver surface of the catalyst. For the purpose of this invention,the anion preferably should not be solely of a chelating type in whichall of the functional groups are tied up by the promoter cation. One ormore of the functional groups should remain relatively free forcomplexation with the silver surface.

The anions which provide the above advantages and which are claimedherein are those derived from unsaturated carboxylic acids, aminoorganicacids, and hydroxybenzoic acids. Examples of suitable unsaturatedcarboxylic acids are acrylic acid, vinylacetic acid, and 5-hexenoicacid. Examples of suitable aminoorganic acids are m-aminobenzoic acid,p-aminobenzoic acid, alpha-aminobutyric acid, and 6-aminocaproic acid.Examples of the hydroxybenzoic acids are m-hydroxybenzoic acid andp-hydroxybenzoic acid.

The method by which the above advantages are achieved comprises forminga compound of the promoter ion which is to be incorporated into thecatalyst and one of the above anions, forming a solution of saidcompound in a suitable solvent, and then contacting the catalyst withthe solution. The concentration of promoter in this solution should bein the range of 1 to 10,000 parts per million. This method can be usedin the production of new catalyst by simply applying the above solutionat the end of the normal catalyst manufacturing process. The method canbe used in the regeneration of used catalyst by merely contacting theused catalyst with the solution.

EXAMPLE

A single sample of aged silver-based ethylene oxide catalyst was used inall of the following experiments. The sample was split into several60-gram portions for identical treatment with cesium salts of thedifferent anions. The treatment procedure consisted of contacting thecatalyst sample with 70 milliliters of a 100 part per million cesium inmethanol solution for two hours, draining, and then drying at 60° C. for20 hours. The 100 parts per million cesium solution was prepared bymixing a 100 parts per million cesium hydroxide solution with anequivalent quantity of the acid according to the equation

    CsOH+HOA→CsOA+H.sub.2 O

where OA represent the anion of the acid.

For this evaluation, acetate provided the base line for a monofunctionalanion. The dried treated catalysts were evaluated by manufacturingethylene oxide in a reactor at a temperature of 400°-500° F. at a flowrate of 200 milliliters per minute of inlet gas with a composition of 7percent oxygen, 8 percent carbon dioxide, 18 percent ethylene, andnitrogen ballast with 1 part per million of ethylene dichloride added asan inhibitor.

The results of these experiments, shown in the following table, provethat the catalysts which were treated with the polyfunctional anionswere superior catalysts to the catalyst which was treated with themonofunctional acetate ion.

                  TABLE 1                                                         ______________________________________                                                                    %                                                                             Selec-                                                                        tivity                                                                        at 1.5%  Temp.                                    Acid         Anion          Δ EO*                                                                            °F.                               ______________________________________                                        Acetic       Acetate        70.4     452                                      Acrylic      Acrylate       72.0     447                                      m-Aminobenzoic                                                                             m-Aminobenzoate                                                                              72.6     458                                      p-Aminobenzoic                                                                             p-Aminobenzoate                                                                              74.0     443                                      gamma-Aminobutyric                                                                         gamma-Aminobutyrate                                                                          72.5     439                                      6-Aminocaproic                                                                             6-Aminocaproate                                                                              70.8     480                                      m-Hydroxybenzoic                                                                           m-Hydroxybenzoate                                                                            71.9     451                                      p-Hydroxybenzoic                                                                           p-Hydroxybenzoate                                                                            72.8     476                                      ______________________________________                                         *The term Selectivity at 1.5% Δ EO means the selectivity at a           productivity of 1.5%.                                                    

I claim:
 1. In a method for improving the selectivity of supportedsilver catalysts by incorporating a promoter therein, the improvementwhich comprises forming a solution of a compound of the promoter and ananion selected from the group consisting of unsaturated carboxylicacids, aminocarboxylic acids, and hydroxybenzoic acids in a solvent andcontacting said catalyst with said solution.
 2. The method of claim 1wherein the concentration of the promoter in the solution is from 1 to10,000 parts per million.